Field of the Invention
The invention relates to a beneficiation process for upgrading mined oil shale prior to retorting for recovery of the oil.
Large deposits of oil shale are found in many locations throughout the world, and extensive efforts have been undertaken to develop oil shale as a source of hydrocarbon products. The term "oil shale" is widely used to refer to a layered sedimentary formation containing an organic material known as kerogen, which may be decomposed by heating to produce gaseous and liquid hydrocarbon products. Such processing of the oil shale may be conducted in place in the deposit (in situ) or the oil shale may be mined by conventional mining methods and the oil shale ore processed by retorting on the surface. In such retorting, particles of mined oil shale are heated over a period of time and to an appropriate temperature to yield gaseous and liquid hydrocarbon fractions. Two examples of such retorts and retorting processes are those described in U.S. Pat. Nos. 3,821,353, and 4,133,741, both of which are assigned to the assignee of the present invention.
Because of the high temperatures required in known retorts and retorting processes for obtaining hydrocarbon values from oil shale, and the resultant need for large amounts of energy to provide such heat, it is desirable to retort as little oil shale as possible to obtain each gallon of oil. It has been found that some oil shale deposits have large amounts of retained water, up to 30% water by weight, in contrast to other known oil shale deposits having less than 4% water by weight. The retorting of oil shales having such a high water content requires increased amounts of heat energy to evaporate the excess retained water.
It has also been determined that this damp oil shale has cohesive tendencies attributable to the presence of bonded finely divided particles of a clay-like material of nominally minus 100 mesh, which has a lower cohesive strength than the larger oil shale particles. Throughout this application and in the appended claims the term "clay" will refer to this clay-like material. This clay also has been found to contain less recoverable hydrocarbons, or oil, per ton than the larger particles of that particular oil shale.
Thus, in retorting this clay-bearing oil shale ore, additional energy must be supplied to the retort to evaporate the additional water and to retort large quantities of the clay which has a lower oil content.